Nutrient and oceanographic changes in the Eastern Equatorial Pacific from the last full Glacial to the Present

Stable isotope measurements of the planktonic foraminiferal species Neogloboquadrina dutertrei and Globigerinoides sacculifer in multiple size fractions were used to examine surface ocean circulation and thermocline nutrient supply in the Eastern Equatorial Pacific (EEP) during the Last Glacial Maximum (LGM). Oxygen isotope values for N. dutertrei in a suite of surface sediment samples are in equilibrium with waters at a consistent position on the temperature gradient from the surface to 100-m water depth. This corresponds to habitats between 40 and 80-m depth, depending on location. The N. dutertrei carbon isotope values follow regional nutrient gradients below 50-m water depth. Combination of oxygen and carbon isotope data for surface sediment samples demonstrates that N. dutertrei maintains a consistent habitat within the lower part of the temperature profile from the surface to 100-m water depth. Mapping isotope values for this species during the LGM in 12 cores across the EEP shows significant changes in regional isotopic gradients. Oxygen isotope values show a regional anomaly, after correction for ice mass change, whose extent depends on corrections made for the temperature shift between the LGM and the Present. Whatever temperature correction used, however, there was a regional change in surface water oxygen isotope values resulting from either dilution of surface waters in the Panama Basin [Mix et al., 1999a] or shifts in eastern boundary current circulation (Mix et al., 1999b). Carbon isotope values for N. dutertrei indicate that regional nutrient gradients seen today were absent during the last full Glacial. The data reveal that thermocline nutrient concentrations decreased during the ice age, in conjunction with the development of a stronger density gradient within the upper water column. This latter change is suggested by relative abundances of selected planktonic foraminifera. In addition, carbon isotope records for G. sacculifer and benthic foraminifera, which carry the global standard signal, differ from that of N. dutertrei, indicating that this species is responding to a local nutrient/circulation change in the thermocline. The timing of carbon isotope shifts during the deglaciation suggests that climate changes in the Antarctic forced oceanic change in the equatorial Pacific.